U.S. Pat. No. 4,297,713 discloses a laser printer having an integral combination of a laser module and an optical system to convert the laser beam of a solid state laser into a collimated beam. The beam is focussed on a photosensitive medium and deflected linewise thereover via a polygonal mirror and a scanning lens. The polygonal mirror and the scanning lens are mounted in a sealed module holder. The lens holder is provided with fit surfaces corresponding to fit surfaces on the module holder. By means of these fit surfaces, the lens holder is mounted and secured in a fixed position against the module holder. The laser holder is mounted against the lens holder via special ball and groove connections. A collimator lens system is mounted in the lens holder. To enable this lens system to be moved with respect to the laser so as to obtain the smallest possible spot on the photosensitive layer, the lens holder has a screwthread. A nut provided with a cylindrical hole is mounted on this screwthread. The lens system is pressed against the nut by a spring and the distance between the lens system and the laser can be varied by turning the nut.
A lens system of the laser module described above has a very small depth of focus, typically about 2-3 .mu.m so that the distance between the lens system and the laser must be set with an accuracy of about 1 .mu.m. To achieve this accuracy, the screwthread must have the smallest pitch possible which in practice, means that the pitch will be about 0.35 mm. It is virtually impossible to obtain practicably a very much finer screwthread.
Another disadvantage of this laser module is that an extremely small angular displacement of the nut (of about 1.degree.) is all that is required to cause a lens displacement of 1 .mu.m. Because of this, there is a need to lock this nut by a second nut to prevent accidental displacement. However, the locking action of the second nut often results in a small angular displacement taking place so that there is a deviation from the previously set distance.
Other similar systems of adjusting a lens system for a laser are shown and described in United Kingdom Pat. No. 345,102 and the article entitled "Passively mode-locked ND:glass laser oscillator optimized for TEM.sub.OO selectivity and long term stability and reliability," in the Review of Scientific Instruments, Vol. 52, No. 6 (June 1981) at pp. 852-57. It would be desirable, therefore, to provide a laser module having a lens system which did not have the above-described disadvantages.